This invention generally relates to a method of supplying power to a continuous ink jet printhead that maintains a proper directionality of a stream of droplets at the beginning of a printing operation.
Many different types of digitally controlled printing systems have been invented, and many types are currently in production. These printing systems use a variety of actuation mechanisms, a variety of marking materials, and a variety of recording media. Examples of digital printing systems in current use include: laser electrophotographic printers; LED electrophotographic printers; dot matrix impact printers; thermal paper printers; film recorders; thermal wax printers; dye diffusion thermal transfer printers; and ink jet printers. However, at present, such electronic printing systems have not significantly replaced mechanical presses, even though this conventional method requires very expensive set up and is seldom commercially viable unless a few thousand copies of a particular page are to be printed. Thus, there is a need for improved digitally controlled printing systems that are able to produce high quality color images at a high speed and low cost using standard paper.
Inkjet printing is a prominent contender in the digitally controlled electronic printing arena because, e.g., of its non-impact low-noise characteristics, its use of plain paper, and its avoidance of toner transfers and fixing. Ink jet printing mechanisms can be categorized as either continuous ink jet or drop on demand inkjet. Continuous inkjet printing dates back to a least 1929. See U.S. Pat. No. 1,941,001 to Hansell.
Conventional continuous ink jets utilize electrostatic charging tunnels that are placed close to the point where the drops are formed in a stream. In this manner individual drops may be charged. The charged drops may be deflected downstream by the presence of deflector plates that have a large potential difference between them. A gutter (sometimes referred to as a xe2x80x9ccatcherxe2x80x9d) may be used to intercept the charged drops, while the uncharged drops are free to strike the recording medium.
A novel continuous inkjet printer is described and claimed in U.S. patent application Ser. No. 08/954,317 filed Oct. 17, 1997, and assigned to the Eastman Kodak Company. Such printers use asymmetric heating in lieu of electrostatic charging tunnels to deflect ink droplets toward desired locations on the recording medium. In this new device, a droplet generator formed from a heater having a selectively-actuated section associated with only a portion of the nozzle bore perimeter is provided for each of the ink nozzle bores. Periodic actuation of the heater element via a train of uniform electrical power pulses creates an asymmetric application of heat to the stream of droplets to control the direction of the stream between a print direction and a non-print direction.
While such continuous ink jet printers have demonstrated many proven advantages over conventional ink jet printers utilizing electrostatic charging tunnels, the inventors have noted certain areas in which such printers may be improved. In particular, the inventors have noted that at the beginning of a printing operation, the first few droplets directed toward the printing medium may be misdirected. While the cause of such droplet misdirection is not entirely understood, the applicants speculate that the principle cause is the non-instantaneous thermal response time of the ink to reach a quasi-equilibrium (operational) temperature since the amount of the drop deflection is directly related to the temperature of the fluid. The duration of the response time is a function of the thermal properties of the heater material, the heater mass, the heater and nozzle geometry as well as the thermal properties of the ink. Any such misdirected droplets can interfere with the objective of obtaining high image 5 quality printing from such devices.
It is an object of the present invention to provide a continuous ink jet method of printing that maximizes print resolution by preventing the misdirection of ink droplets at the beginning of a printing operation.
It is another object of the present invention to provide a continuous ink jet printing method that prevents ink drop misdirection which may be used in an asymmetric heat-type printer without the need for making structural changes in such a printer.
Both of these objects are realized by the method of the invention, which generally comprises the step of supplying power to the heating element that is adjacent to the nozzle at a higher level than normal during the ejection of the first few ink droplets from the nozzle.
During normal printing operations, power pulses conducted to the heating element adjacent to each nozzle are comprised of a train of pulses having a constant amplitude, width, and frequency. In the method of the invention, at least one of the electrical characteristics of the pulse train is changed so that power is supplied to the heating element at a higher level than the constant operational level. Accordingly, the initial pulse or pulses have either a greater amplitude or width or a different frequency than the electrical pulses used during the balance of the printing operation.
In the embodiment of the method wherein the amplitude of the initial electrical pulses is increased, at least the first power pulse may have an amplitude between about 10% and 60% greater than the amplitude of a normal, operational power pulse. Alternatively, at least the first power pulse may have a width that is between about 60% and 300% more than the width of an operational power pulse. In still another embodiment of the method, the time interval between the first two pulses may be reduced to between about 25% and 50% of the time interval between subsequent operational power pulses. In all of the preferred embodiments, no more than about the first four power pulses have one of a greater amplitude, width, or a higher frequency than the balance of the power pulses used during the printing operation.
In the embodiment of the method wherein the first power pulse has an amplitude of between about 10% and 50% greater than the amplitude of an operational power pulse, the time period between the second power pulse and a third power pulse may be between about 10% and 100% greater than the time period associated with the operational power pulses.
In all of the embodiments of the invention, the method may be implemented simply by adjusting or reprogramming the shape or frequency of the power pulses generated by the power supply of the ink jet printer. The method is capable of substantially reducing, if not eliminating entirely, spurious ink drop deflection occurring at the beginning of a printing operation. Hence, the resolution of the final printing product is improved.